The present invention is directed to a method of forming expansion dowels with at least an axially extending expansion region provided with axially extending slots and a bore tapering inwardly in the expansion region towards a leading end of the dowel.
Expansion dowels of this general type are anchored in a foundation, such as concrete, rock, masonry and the like by radially widening an expansion sleeve. Such radial widening determines the anchoring values to be achieved, since a connection is created between the expansion sleeve and the foundation affording on one hand a frictional lock and in certain cases a positive locking feature. The extent of a frictionally locked connection is in particular influenced by the amount of the surface available for this purpose on the expansion sleeve, that is, a small surface on the expansion sleeve available for friction locking affords lower anchoring values.
An expansion dowel is known from DE-PS 21 52 729 having a radially widenable expansion sleeve. The expansion region of the expansion sleeve has axially extending slots for affording the radial widening with tabs formed between the slots. The expansion sleeve is radially widened by driving an expansion member into the bore in the expansion sleeve which tapers inwardly toward its leading end, so that the tabs move radially outwardly. After the radial widening has occurred, that is, when the expansion dowel has been completely set, a member, such as a bolt, a threaded rod or the like can be fastened in the trailing end of the expansion sleeve by load engaging means in the shape of a thread located at the trailing end directed away from the expansion region.
The above-mentioned known expansion dowel can be provided with four axially extending slots. These slots are formed by a machine cutting operation so that they have a specific width transversely of the axial direction, depending on the tool used, such as the side milling cutter. Since the width of the axially extending slots depends on the width of the removing tool used, for economic reasons the width of the slots is the same for all sizes of expansion dowels. As a result, a considerable share of the surface of the expansion dowel is consumed by the axially extending slots, particularly in the case of smaller diameter expansion dowels. Such share of the surface is not available for affording the connection with the foundation, whereby the achievable anchoring values are reduced. Therefore, with smaller diameter expansion dowels, the share of the surface available for providing a connection with the foundation is reduced by up to 20%. This situation is further exacerbated, since in the radial expansion step the axially extending slots are widened.